When ships such as tankers, large cargo ships, etc. sail with no oil or cargoes or with a small amount thereof being loaded, generally they sail with ballast water stored in a ballast tank to ensure the stability and balance of the ships. Usually, the ballast water is loaded thereunto by pumping up, for example, seawater at the port of discharge and is discharged at the port of loading. Thus, since, for example, seawater of the port of discharge is used for the ballast water, the ballast water contains, for example, aquatic microorganisms living around the port of discharge, and the aquatic microorganisms are discharged together with the ballast water at the port of loading. Therefore, when being loaded into a ship, ballast water is treated, for example, to separate/remove foreign matters contained in a liquid taken in, using a centrifugal separation method (for example, Patent Document 1).